I don't have the answer but I bet the OP's pants turned brown when he heard that bang.
I can only think that raw fuel must have somehow found its way into the OAP.
That could indeed happen if the PCM went into a COT prevention event (catalytic converter overheat...PCM will increase fuel amount into the cylinders on the specific bank to attempt to cool the overheated catalytic converter substrate in addition to cutting ignition timing as noted by the exhaust temp inferred from the rear O2 sensor heater circuit readout of the affected cat...OEM tune set @ 1,650*F) & since the Roush catback exhaust uses resonators in mid pipe section they can trap excess fuel vapors in them (these are an expansive type of sound control.....it's in the picture just ahead of the damage) & it can flash off if the resonator got hot enough & enough free unburnt O2 was present in the exhaust to promote the "rapid expansion" as these are close enough to the cats to retain enough heat from them (cats at speed under an OEM tune put off approx 1,200*F-1,300*F but if tuned for performance can get in excess of 1,400*F-1,500*F + depending on conditions.....mine for example currently run around 1,430*F-1,450*F at speed @ FBO\cammed\tuned NA 3V thru Kooks Hi Flow Race cats....highest I've read thru a scan tool was 1,437*F) to ignite the unburnt air\fuel mix. Engine misfiring at speed can set this up as well if it is severe enough (dumping unburnt air\fuel mix into exhaust causing cats to overheat from having to burn off the excess air\fuel thus set off the PCM to go into COT prevention) but if this was the case the PCM should have set the MIL flashing......if the misfiring counts exceeded the threshold. Might check the PCM for any pending codes\Mode 6 Misfire Monitor data for any recorded cylinder misfires that could shed some light.........
The piping shows signs of overpressure from within & the materials most likely came from the Roush resonator...........
If this was the std OEM catback exhaust layout (nothing installed in mid exhaust piping post cats...mufflers are mounted at rear of car at end of exhaust piping) then this would indeed be very near impossible as the exhaust temps would have greatly dissipated by the time it got to the mufflers (next place to potentially trap any unburnt fuel vapors) but w\ the Roush catback exhaust system (or any mid muffler\resonator catback exhaust system w\ cats still being used w\ the rear O2 sensors still active so the PCM can go into COT if it deems necessary) this can occur if the conditions are right.
Not saying that this is the culprit that caused this particular instance w\ the OP's exhaust but this is possible w\ these S197's
that are still running cats w\ the rear O2 sensors active w\ a resonator\muffler mounted in mid pipe section.
Something to keep in mind for those of us who are still running cats & use a mid mounted resonator\muffler catback exhaust. Also why you also should be mindful of how your engine is tuned when cats are still being used to stay out of COT. Those that are running a FF exhaust setup.......won't typically run into this....unless the rear O2 sensors are still active & COT is still active in the tune then this still "could" happen depending on how hot the exhaust is (think FI at this point) at the mid mounted resonator\muffler....
Agree w\ Mr Awesome987.....OP needs to pull & inspect all of his exhaust to get the full extent of the damage then once any repairs are made to the exhaust I would have the engine & engine management systems checked to ensure all is good.....IMHO something had to get out of spec to cause that to happen......Roush may warranty\replace this part but be prepared if they don't.
PS--For all those that like the crackles\pops from the exhaust upon decel....that's from the engine pumping unburnt O2 into a heated exhaust w\ some unburnt fuel vapors still present so don't think it's impossible.....some folks tune their cars to do this............